Various control methodology and systems have been employed in conjunction with internal combustion engines so as to improve their performance, particularly in the areas of fuel economy and exhaust emission control. One of the more effective types of controls is a so-called "feedback" control. With this type of control, a basic air/fuel ratio is set for the engine. Adjustments are then made from the basic setting based upon the output of a sensor that senses the air/fuel ratio in the combustion chamber in order to bring the air/fuel ratio into the desired range.
Normally, the type of sensor employed for such feedback controls is an oxygen (O.sub.2) sensor which provides an electrical output signal. Generally, when the output signal voltage is high, little oxygen is present in the exhaust thus indicating that a charge which was rich in fuel was been supplied. On the other hand, when the output signal voltage is low, substantial amounts of oxygen are present in the exhaust, thus indicating that a charge which was rich in air was been supplied.
This type of sensor is normally associated with a wave-forming circuit which manipulates the output of the sensor to indicate an "ON" signal when the voltage of the output signal exceeds a reference voltage (i.e. a signal which results when the supplied charge is rich in fuel). On the other hand, the circuit manipulates the signal to indicate that the sensor is "OFF" when the voltage of the output signal does not exceed the reference voltage (i.e. a signal which results when the supplied charge is rich in air).
The control operates on a feedback-control principle, continuously making corrections to accommodate deviations from the desired air/fuel ratio. Adjustments are made in stepped intervals until the sensor output goes to the opposite sense from its previous signal. For example, if the mixture is too rich in fuel (i.e. the sensor signal is "ON"), then lean adjustments are made until the mixture strength is sensed to be lean (i.e. the sensor signal turns "OFF"). Adjustments are then made back into the rich direction in order to try to maintain the desired ratio.
Most commonly, the oxygen sensor is of the type which utilizes inner and outer platinum or platinum coated electrodes. The use of platinum as the electrode portions of the sensor is advantageous for serving the purposes of the electrical activity associated with the electrodes. On the other hand, the platinum acts as a catalyst, catalyzing exhaust. For example, oxygen remaining in the exhaust may be catalyzed with carbon monoxide at the platinum electrode interface, creating carbon dioxide. While the effect of the platinum in improving exhaust gas emissions may be advantageous, the oxygen content of the gas being sensed can be affected to a degree which causes the sensor to provide inaccurate data, causing the control to adjust the air/fuel ratio erroneously.
For example, while the actual oxygen content of the exhaust system may correspond to an air rich air/fuel charge such that the actual signal from the sensor should indicate that the sensor is "OFF," the above-described effect may cause the sensor to indicate little oxygen is present (i.e. as if a fuel rich charge had been supplied) by an "ON" signal. In that instance, the feedback control is arranged to adjust the air/fuel ratio in the fuel-rich direction in response to the "ON" signal even though the mixture is already fuel-rich.
It is, therefore, a principal object of this invention to provide an improved feedback control system for an engine, the feedback control including an oxygen sensor.